Growing body research shows that SARS-CoV-2, the virus that causes COVID-19, can spread from person to person through the air. Indoor spaces with poor ventilation in areas where the virus is present are very dangerous.
In the fictional world of “Star Trek”, public health officials and first responders would be able to directly determine if a space has a dangerous concentration of airborne viruses, and all other pathogens, simply by weighing a tricorder.
That technology, invented 60 years ago, is still firmly in the realm of fiction. However, devices that can quickly detect certain pathogens in the air – including SARS-CoV-2 – are in the works in several research laboratories.
The air we breathe
Detection of the presence of airborne virus particles is complicated by the mixing of other airborne particles. The atmosphere contains a large number of floating particles, a significant fraction of which is biological. Typically, you inhale with each breath about a thousand biological particles.
These bioaerosols contain living and dead organisms, including viruses, bacteria, fungi, pollen and plant and animal waste. Viruses are the smallest of these particles. They vary in size from 10 to 300 nanometers, as millionths of a millimeter. In contrast, red blood cells average about 6 to 8 microns, or 6,000 to 8,000 nanometers, in diameter. Bacteria range from 1 to 4 microns and fungi from 5 to 10 microns. Plant and animal warfare is generally greater than 10 microns.
Most of these biological particles are not health care because most pieces are plants and animals, including humans. However, it only takes a small number of dangerous microbes to produce a pandemic.
IDs of bad news microbes
To understand the potential threat posed by bioaerosols, it is important to identify the small fraction of problematic or pathogenic microbes present from all bioaerosols. Bioaerosol identification begins with capturing biological particles from the air, typically by collecting particles on a filter, in a liquid vial or on hydrogels.
Often, researchers transfer the collected bioaerosols to a culture medium designed to support microbial growth. How the microbes respond to a specific culture medium – the size, shape, color and growth rate of the microbial colony – can indicate the microbial species.
This process can take several days to weeks, and is often ineffective. It turns out that scientists using this approach can only identify about 1% of airborne microbes.
More often, scientists rely on gene-based assays to collect viruses and other microorganisms collected in air samples. One popular technique for gene-based analysis is polymerase chain reaction (PCR), which uses an enzymatic reaction to make multiple copies of a specific gene as part of a gene so that the genetic sequence – DNA or RNA – can be detected in a sample. A PCR test can be designed to specify gene sequences specific for a microorganism, so that detecting the sequence is tantamount to identifying the microorganism.
This technique is currently the gold standard for detecting the presence of SARS-CoV-2 from nasal swab samples. PCR-based methods are very accurate in identifying pathogens.
Next-generation sequencing technology makes it possible to track entire genomes of organisms in rapid sequences. Using these techniques, researchers now have the ability to understand the entire population of microorganisms – their diversity and abundance – in the air.
Rapid detection
Despite these advances, much work remains to be done to directly identify the presence of pathogens in the air. Current microbial identification techniques are expensive, require special equipment, and involve lengthy processing steps. They also cannot detect any species from small amounts of genetic material.
Recent advances, however, provide some promise for the development of sensors that can provide rapid information about bioaerosols.
One approach uses laser-induced fluorescence. In this technique, particles are illuminated with light of a certain color or wavelength, and only biological particles react by fluorescence, or emitting light. This technique can be used to identify and quantify the presence of biological particles in the air in real time, but it does not distinguish between a safe and a harmful microbe.
Jan Pavelka / Wikimedia Commons, CC BY-SA
Another advance is with mass spectrometry for bioaerosol detection. In this technique, a single bioaerosol particle is blown apart with a laser and the molecular fragments are immediately analyzed to determine the molecular composition of the particles. Researchers also use Raman spectroscopy-based sensors. Raman spectroscopy can identify molecular composition from light reflected from samples without destroying the samples.
Big challenge in a small package
These techniques promote direct detection and identification of airborne bacteria and fungi, but they are less efficient at detecting viruses, including SARS-CoV-2. This is primarily because viruses are very small, making it difficult to collect them with air samples and difficult to perform PCR analysis, given the small amount of DNA / RNA.
Researchers are working to address the limitations of detection of viruses in the air. In our laboratory at Clarkson University, we have developed a low cost bioaerosol sensor and collector for wide scale bioaerosol sampling. This sampler-operated sampler uses a micro-large source of high voltage to ionize and collect ionizing viruses, bacteria and fungi on a surface. Ionization gives the biological particles an electric charge. Giving the collection surface over the opposite charge causes the particles to surface.
Courtesy of Oxford Nanopore Technologies
Samples from our collector can be analyzed with new portable DNA / RNA sequencers, enabling real-time bioaerosol detection using inexpensive, hand-held equipment.
Where’s my tricorder?
These advances could soon make it possible to detect a known pathogen, such as SARS-CoV-2, with a portable device. But they are still far from being a tricorder.
[Deep knowledge, daily. Sign up for The Conversation’s newsletter.]For one, they require relatively high levels of a pathogen for detection. Being able to identify a virus like SARS-CoV-2 at lower levels that are nevertheless sufficient for transmitting diseases, will develop sensors with lower detection limits. In addition, these sensors can only be adapted to detect specific pathogens, not to scan for all possible pathogens.
Although the equivalent of the tricorder in “Star Trek” is not around the corner, the need for such a device has never been greater. Now is a suitable time for the emergence of new sensing techniques piggy-backing on the dramatic advances being made in the fields of electronics, computing and bioinformatics. If the following new pathogen appears, it would be nice to have a tricorder handy.
